Measuring the coagulation time of blood is of great medical interest. Coagulation disorders can lead to severe physiological disturbances such as abundant bleeding during surgery or wounds that bleed excessively (coagulation time too long), or such as thromboses or embolisms (coagulation time too short). Very many diseases, pathologies, medical treatments, or therapy follow-ups require such coagulation time to be checked on a more or less regular basis. Frequently this involves blood being taken from a vein, which requires intervention by a specialist. The sample taken is then analyzed in a specialized laboratory, where various different tests can be performed for measuring coagulation time, depending on the pathology or the treatment of the patient under analysis. Mention can be made in particular of the following: Quick's prothrombin time (PT or QT); activated cephalin time (ACT); and kaolin cephalin time (TCK). Frequently, it is not directly the absolute values of these coagulation times that are of interest for clinicians, but rather their ratios relative to reference times for the same test, performed on a reference sample made up from a pool (or mixture) of fifty or more samples from patients deemed to be normal. In addition, the measured coagulation time depends on the physical method used for characterizing the coagulation phenomenon, on the way in which the sample is mixed with the coagulation factor under study (mixing time), and on the reagent used for triggering the reaction. It is therefore common practice to apply a correction in order to obtain a result that is independent of these factors. By way of example, mention can be made of the international normalized ratio (INR) that is calculated on the basis of the prothrombin time divided by the reference time and raised to the power ISI (international sensitivity index) as given by the manufacturer of the batch of reagents used for implementing the tests.
Conventionally, measuring blood coagulation time involves the following steps.
In a first step, the blood is taken from the patient in a tube that contains a suitable anticoagulant giving a reasonable amount of time for transport between the place where the sample is taken and the place where it is analyzed (a minimum of several minutes) without the sample coagulating.
Thereafter, the plasma is extracted from the blood sample by centrifuging and is mixed in the appropriate proportions with various reagents needed for inhibiting the anticoagulant that was used when taking the sample (e.g. calcium ions), and with the reagents needed for triggering coagulation (e.g. thromboplastin), depending on the factor under study.
Finally, coagulation time proper is measured with the help of various kinds of equipment.
Document U.S. Pat. No. 4,252,536 describes an optical device and method for measuring coagulation time. That device and method are based on variation in the intensity of a detected optical signal as a result of a modification in diffusion through a plasma sample during the formation of a coagulated blood clot.
Document U.S. Pat. No. 3,635,678 describes another method in common use, consisting in introducing a magnetic bead into the plasma sample, and putting the bead into oscillatory motion with the help of an external magnet or electromagnet. The movement of the bead is observed optically. The measured time at the end of which the bead freezes in the plasma, being held stationary by the clot that is forming, corresponds to the coagulation time.
Those methods are not suitable for use with whole blood, since it is too opaque.
Document U.S. Pat. No. 6,352,630 describes an electrochemical method of measuring coagulation time. Implementing that method requires a consumable comprising electrodes that come into contact with the biological sample, equipment that serves to relay the electrical contacts on the consumable, and the addition to said sample of electrochemical agents enabling measurement to be performed.
The article by Yann Piederriére et al. entitled “Evaluation of blood plasma coagulation dynamics by speckle analysis” describes two methods of studying the dynamics of blood coagulation by analyzing laser speckle.
Those methods are based on the observation that the particles (platelets, proteins) in suspension in the plasma diffract and diffuse light; if the plasma sample is illuminated by a laser beam that is spatially and temporally coherent, a speckle pattern appears. Because of the brownian movement of the particles, the pattern varies over time so long as the plasma remains liquid, and then freezes once the clot has formed.
In the first method described in the above article, the light intensity corresponding to one point of the speckle pattern is registered as a function of time. That method is quite difficult to implement: the transparency of the plasma decreases during the coagulation process; it is therefore necessary to increase the illumination light intensity over time, or to illuminate the sample rather strongly throughout the duration of acquisition, at the risk of saturating the detector. Furthermore, since the light intensity at only one point is taken into consideration, the optical signal is weak, thereby implying an unsatisfactory signal-to-noise ratio, unless the illumination light intensity used is relatively high.
The second method described in that article involves acquiring a time series of images of the speckle pattern, and in determining the contrast in each image. Before the clot forms, the brownian movement of the particles in suspension “scrambles” the image, thereby reducing their contrast. Contrast increases when the speckle pattern freezes as a result of the plasma coagulating. As admitted by the authors, that method does not enable coagulation time to be determined accurately.
All of the above-described methods make use of plasma, and cannot operate with whole blood. They therefore require a prior step of fractioning the blood, and that can be performed only in a specialized laboratory.